Different wireless location technologies and systems for determining a location of a mobile transmitter or receiver of electromagnetic waves, and thereby of a person or object, carrying or mounted with the transmitter or receiver are known. Among well known and ubiquitous systems that provide “tracking” services that determine locations for people and objects are, by way of example, the mobile phone network systems and global navigation satellite systems, of which the global positioning system (GPS) is perhaps the best known. For convenience of discussion, a transmitter and/or receiver, and/or a device in which it is housed, and/or its bearer, for which a location technology or system, hereinafter also a tracking system or tracking technology, determines locations may be distinguished as being a “target”, used as a modifier or noun, of the tracking technology or system.
In the GPS tracking system, a GPS receiver, such as is commonly available for locating vehicles and persons, receives electromagnetic waves encoding synchronized clock signals from at least three GPS satellites. The clock signals are used to determine transit times of the electromagnetic waves from the satellites. The speed of light and transit times from the satellites are used to determine where on the face of the globe the receiver, and thereby the person or object bearing the receiver, is located. Mobile phone networks use signals transmitted by synchronized base station transmitters from different cells in the network to mobile terminals, such as mobile phones, personal digital assistants (PDA), and laptop computers, to track locations of the terminals. Locations of a mobile terminal may be provided responsive to differences in time of arrival, or differences in strength, of signals transmitted to the terminal by the base station transmitters.
It is noted that whereas in the above discussion, a single “target” receiver is described as being located by a tracking system responsive to electromagnetic signals transmitted from a plurality of transmitters, location technologies are of course not limited to locating a receiver using signals from a plurality of transmitters. Location technologies may generally be used in “reverse”, with a single transmitter transmitting signals to three or four receivers to provide a location for the transmitter.
In general, functioning of wireless tracking systems, in determining locations of targets responsive to transmission of electromagnetic waves, is adversely affected by objects, terrain features, and/or weather artifacts that interfere with propagation of the waves. For example, most of the systems are adversely affected by “multipath signaling”. In multipath signaling, energy from a same signal transmitted by a transmitter to a receiver is reflected from, or refracted by, objects, terrain features, and/or atmospheric artifacts present in a path along which the energy propagates so that portions of the energy travel by more than one path to the receiver. Different amounts of energy from the same signal therefore arrive at the receiver at different times. As a result, signal fidelity, and measurements of signal transit times and/or signal strength, may be compromised. Accuracy of locations provided by mobile phone systems are often substantially degraded by multipath effects. And, whereas GPS based location systems can be configured to provide determinations of locations of a receiver that are accurate to a few tens of centimeters, they generally do not function for locations of a receiver unless unencumbered lines of sight from the receiver location to at least three GPS satellites is available.
Tracking systems that use low frequency magnetic fields have been developed for locating a target under conditions for which masking or shielding of electromagnetic waves by objects, terrain features, and/or atmospheric artifacts make locating the target using electromagnetic wave tracking systems unreliable. Static and low frequency magnetic fields are substantially less affected by common masking or shielding artifacts than are electromagnetic waves. For example, magnetic tracking systems may be used to locate miners in mines and drilling rig drill tips in bore holes.